Electric oil pump

ABSTRACT

An electric oil pump including an electric motor, a first pump element that is driven to suck and discharge oil by output torque of the electric motor, a second pump element that is driven to suck and discharge the oil by the output torque of the electric motor, and a torque transmission switching member disposed on a torque transmission path extending between the electric motor and the second pump element. The torque transmission switching member is actuated to allow torque transmission between the electric motor and the second pump element when viscosity of the oil is not higher than a predetermined value, and interrupt the torque transmission between the electric motor and the second pump element when viscosity of the oil is higher than the predetermined value.

The present invention relates to an electric oil pump.

Japanese Patent Application Unexamined Publication No. 2007-009790 discloses an electric oil pump of a trochoid type which includes a pump rotor and an outer rotor. In the electric oil pump of this conventional art, a drive shaft fixed to the pump rotor is coupled to an electric motor so that the electric oil pump is driven by the electric motor.

SUMMARY OF THE INVENTION

Since usually a gear pump of a trochoid type is a fixed volume pump that discharges a constant amount of oil per unit rotation, output torque of an electric motor which is necessary to ensure a predetermined amount of oil to be discharged is increased as a viscosity of the oil becomes higher. As a result, current consumption of the electric motor is also increased. Further, an amount of heat generated from the electric motor is increased in proportion to the square of an amount of current consumption of the electric motor. Therefore, there occurs a problem of heat generation from the electric motor under a low temperature environmental condition with a high viscosity of oil.

It is an object of the present invention to provide an electric oil pump capable of suppressing heat generation from an electric motor under a low temperature environmental condition.

In one aspect of the present invention, there is provided an electric oil pump including:

an electric motor;

a first pump element that is driven to suck and discharge oil by output torque of the electric motor;

a second pump element that is driven to suck and discharge the oil by the output torque of the electric motor; and

a torque transmission switching member disposed on a torque transmission path that extends between the electric motor and the second pump element, the torque transmission switching member being actuated to allow torque transmission between the electric motor and the second pump element when viscosity of the oil is not higher than a predetermined value, and interrupt the torque transmission between the electric motor and the second pump element when viscosity of the oil is higher than the predetermined value.

The electric oil pump according to the present invention can suppress heat generation from an electric motor under a low temperature environmental condition.

The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an electric oil pump according to a first embodiment of the present invention.

FIG. 2A and FIG. 2B are schematic diagrams showing a clutch of the electric oil pump according to the first embodiment of the present invention.

FIG. 3 is a sectional view taken along line S2-S2 as shown in FIG. 1.

FIG. 4 is a sectional view taken along line S3-S3 as shown in FIG. 1.

FIG. 5 is an exploded perspective view of essential parts of the electric oil pump according to the first embodiment of the present invention.

FIG. 6A is a characteristic diagram showing a relationship between oil viscosity and torque in an electric oil pump.

FIG. 6B is a characteristic diagram showing a relationship between electric current and torque in an electric oil pump.

FIG. 7 is a characteristic diagram showing a relationship between torque and rotation and a relationship between torque and electric current in the electric oil pump according to the first embodiment of the present invention.

FIG. 8A and FIG. 8B are diagrams showing an electric oil pump of a comparative example in which a first pump element and a second pump element are arranged in series with respect to an oil circulation path.

FIG. 9A and FIG. 9B are diagrams showing the electric oil pump according to the first embodiment of the present invention in which a first pump element and a second pump element are arranged in parallel with each other with respect to an oil circulation path.

FIG. 10 is a schematic diagram showing an electric oil pump according to a second embodiment of the present invention.

FIG. 11 is a schematic diagram showing an electric oil pump according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following, an electric oil pump according to respective embodiments of the present invention is explained by referring to the accompanying drawings.

First Embodiment

FIG. 1 is a schematic diagram showing an electric oil pump according to a first embodiment. The electric oil pump according to the first embodiment is a pump that is mounted to an automatic transmission of a vehicle having an idle stop function. The automatic transmission is a belt-drive continuously variable transmission. A main pump that is driven by an engine is separately provided. When the engine is stopped by idle stop control, a sufficient hydraulic pressure that is produced by the main pump cannot be ensured. Further, in a case where a drop in hydraulic pressure is caused due to oil leak from friction engagement elements and pulleys in the belt-drive continuously variable transmission, it will take much time to ensure a hydraulic pressure necessary upon restarting the vehicle, thereby causing deterioration in drivability. In addition, in a case where operation changeover between a motor and an engine is carried out by a clutch as in a hybrid vehicle, torque that can be transmitted is reduced at the occurrence of lack of cooling oil, thereby causing deterioration in drivability at the times of vehicle starting, accelerating, etc. In order to avoid these problems, the electric oil pump according to this embodiment which is capable of discharging a hydraulic pressure regardless of an operating condition of the engine is provided separately from the main pump. With the provision of the electric oil pump according to this embodiment, drivability at the times of restarting the engine and restarting the vehicle can be enhanced by ensuring an amount of the hydraulic pressure which is leaked from the friction engagement elements and the pulleys.

The electric oil pump according to the first embodiment includes first pump element 1, second pump element 2, electric motor 3, casing 4, driver 5, heat sink 6, and clutch (i.e., a torque transmission switching member) 7. First pump element 1 is driven to suck and discharge oil by output torque of electric motor 3. First pump element 1 is a trochoid pump element including pump rotor 11 having an externally toothed gear (hereinafter referred to as “an external gear”) and outer rotor 12 having an internally toothed gear (hereinafter referred to as “an internal gear”). Second pump element 2 is driven to suck and discharge oil by the output torque of electric motor 3. Second pump element 2 is a trochoid pump element including pump rotor 13 having an externally toothed gear (hereinafter referred to as “an external gear”) and outer rotor 14 having an internally toothed gear (hereinafter referred to as “an internal gear”). Electric motor 3 includes motor rotor 16 connected with first shaft 15, and stator 17.

Casing 4 is mounted to an automatic transmission case that accommodates the automatic transmission. Casing 4 includes first casing 18 that accommodates first pump element 1 and electric motor 3, second casing 19 that accommodates second pump element 2, and cover member 20 that covers an opening of second casing 19. First casing 18 includes first pump accommodating portion 21 in which first pump element 1 is accommodated, and motor accommodating portion 22 in which electric motor 3 is accommodated. Specifically, pump rotor 11 of first pump element 1 is supported so as to be rotatable about a rotation axis in first pump accommodating portion 21. Stator 17 is fixedly supported in motor accommodating portion 22. Bearing portion 23 in which first shaft 15 is rotatably supported is disposed between first pump accommodating portion 21 and motor accommodating portion 22. Second casing 19 includes second pump accommodating portion 24 in which second pump element 2 is accommodated, and clutch accommodating portion 25 in which clutch 7 is accommodated. Pump rotor 13 of second pump element 2 is supported so as to be rotatable about a rotation axis in second pump accommodating portion 24. Bearing portion 27 in which second shaft 26 is rotatably supported is disposed between second pump accommodating portion 24 and clutch accommodating portion 25. Second shaft 26 is connected with pump rotor 13 of second pump element 2. Cover member 20 serves to close one end of second pump accommodating portion 24 of second casing 19, and has bearing portion 28 in which a tip end portion of second shaft 26 is rotatably supported as explained later.

Driver 5 is fixed to first casing 18, and supplies a driving current to stator 17 of electric motor 3 in accordance with a command outputted from a control unit (not shown) that controls electric motor 3. Heat sink 6 is mounted to driver 5, and serves to cool driver 5 by heat radiation. Clutch 7 is disposed between first shaft 15 and second shaft 26. Clutch 7 is a torque sensitive clutch that is engaged when torque is applied between first shaft 15 and second shaft 26 is not more than a set torque, and is disengaged (released) when the torque applied between first shaft 15 and second shaft 26 is more than the set torque. The set torque is determined as a value of torque which is applied between first shaft 15 and second shaft 26 when oil viscosity becomes a predetermined value. The predetermined value of oil viscosity is determined as a value of oil viscosity at which current consumption of electric motor 3 becomes the allowable current when electric motor 3 is driven while holding clutch 7 in the engagement state.

FIG. 2A and FIG. 2B are schematic diagrams showing clutch 7. As shown in FIG. 2A and FIG. 2B, clutch 7 includes outer cylindrical member 7 a, inner cylindrical member 7 b, engaging portion 7 c, ball 7 d, and coil spring 7 e. Outer cylindrical member 7 a is connected to first shaft 15 through connection hole 34 as explained later, and makes unitary rotation with first shaft 15. Inner cylindrical member 7 b is connected to second shaft 26, and makes unitary rotation with second shaft 26. Engaging portion 7 c is disposed on an inner periphery of outer ring 7 a. Ball 7 d is mounted into groove 7 f formed in an outer periphery of inner cylindrical member 7 b, via coil spring 7 e. Ball 7 d is pressed onto an inclined surface of engaging portion 7 c by a biasing force of coil spring 7 e. In a case where the torque applied between outer cylindrical member 7 a and inner cylindrical member 7 b when outer cylindrical member 7 a is rotated is not more than the set torque, the biasing force of coil spring 7 e which acts on ball 7 d to push ball 7 d out of groove 7 f is larger than the force that acts on ball 7 d from engaging portion 7 c to push ball 7 d into groove 7 f. As a result, engaging portion 7 c and ball 7 d are engaged with each other in a circumferential direction of clutch 7, so that the torque is transmitted from outer cylindrical member 7 a to inner cylindrical member 7 b. On the other hand, in a case where the torque applied between outer cylindrical member 7 a and inner cylindrical member 7 b when outer cylindrical member 7 a is rotated is more than the set torque, the force that acts on ball 7 d from engaging portion 7 c to push ball 7 d into groove 7 f becomes larger than the force of coil spring 7 e which acts on ball 7 d to push ball 7 d out of groove 7 f. Therefore, ball 7 d is held within groove 7 f, so that the torque is not transmitted from outer cylindrical member 7 a to inner cylindrical member 7 b.

Outer casing 18 is formed with suction port (oil suction port) 39 and discharge port (oil discharge port) 40 of first pump element 1 which are disposed on a radial outside of outer rotor 12. Suction port 39 and discharge port 40 are symmetric with respect to a line extending between the rotation axis of pump rotor 11 and the rotation axis of outer rotor 12. Suction port 39 is formed to face to respective suction side volume chambers having a volume that is increasing to thereby produce a negative pressure in the suction side volume chambers, among respective volume chambers formed between the external gear of pump rotor 11 and the internal gear of outer rotor 12. Discharge port 40 is formed to face to respective discharge side volume chambers having a volume that is reducing to thereby raise an internal pressure in the discharge side volume chambers, among the respective volume chambers. FIG. 3 is a sectional view taken along line S2-S2 shown in FIG. 1. As shown in FIG. 3, first casing 18 is formed with suction oil passage 41 and discharge oil passage 42 which are disposed on an outer circumferential side of first pump element 1. Suction oil passage 41 is communicated with suction port 39 and axial oil passage 47 that extends in first casing 18 in an axial direction of first pump element 1. Axial oil passage 47 is communicated with an oil inlet opened into an oil pan (not shown). Discharge oil passage 42 is communicated with discharge port 40 and axial oil passage 48 that extends in first casing 18 in the axial direction of first pump element 1. Axial oil passage 48 is communicated with a control valve unit (not shown).

Second casing 19 is formed with suction port (oil suction port) 43 and discharge port (oil discharge port) 44 of second pump element 2 which are disposed on a radial outside of outer rotor 14. Suction port 43 and discharge port 44 are symmetric with respect to a line extending between the rotation axis of pump rotor 13 and the rotation axis of outer rotor 14. Suction port 43 is formed to face to respective suction side volume chambers having a volume that is increasing to thereby produce a negative pressure in the suction side volume chambers, among respective volume chambers formed between the external gear of pump rotor 13 and the internal gear of outer rotor 14. Discharge port 44 is formed to face to respective discharge side volume chambers having a volume that is reducing to thereby raise an internal pressure in the discharge side volume chambers, among the respective volume chambers. FIG. 4 is a sectional view taken along line S3-S3 shown in FIG. 1. As shown in FIG. 4, second casing 19 is formed with suction oil passage 45 and discharge oil passage 46 which are disposed on an outer circumferential side of second pump element 2. Suction oil passage 45 is communicated with suction port 43 and axial oil passage 49 that extends in second casing 19 in an axial direction of second pump element 2. Axial oil passage 49 is merged with axial oil passage 47 of first casing 18, and is communicated with an oil inlet opened into an oil pan (not shown). Discharge oil passage 46 is communicated with discharge port 44 and axial oil passage 50 that extends in second casing 19 in the axial direction of second pump element 2. Axial oil passage 50 is merged with axial oil passage 48 of first casing 18, and is communicated with the control valve unit (not shown).

FIG. 5 is an exploded perspective view of essential parts of the electric oil pump. As shown in FIG. 5, pump rotor 11 of first pump element 1 has connection hole 30 on a central portion thereof. Connection hole 30 has two-face portion 29 defining a width across flats between two flat faces that are spaced from and opposed to each other. First shaft 15 has rotor engaging portion 31 on a tip end portion thereof which is engaged with two-face portion 29 of connection hole 30. First shaft 15 also has clutch engaging portion 32 on a tip end thereof. Clutch engaging portion 32 includes fitting portion 32 a that is fitted to two-face portion 33 of connection hole 34 of clutch 7. Two-face portion 33 defines a width across flats between two flat faces of connection hole 34 which are spaced from and opposed to each other. Second shaft 26 has rotor engaging portion 35 on a tip end portion thereof which is engaged with two-face portion 37 of connection hole 38 of pump rotor 13 of second pump element 2. Two-face portion 37 defines a width across flats between two flat faces of connection hole 38 which are spaced from and opposed to each other. Second shaft 26 also has support shaft 36 that is supported by bearing portion 28 of cover member 20.

Functions of the electric oil pump according to the first embodiment will be explained hereinafter.

[Operation of Electric Oil Pump]

When first shaft 15 is driven to rotate by applying electric current to electric motor 3, pump rotor 11 integrally formed with first shaft 15 is rotated. At this time, in a case where oil viscosity is not higher than the predetermined value, the torque applied to clutch 7 disposed between first shaft 15 and second shaft 26 is not larger than the set torque. Therefore, clutch 7 is kept in the engagement state to thereby allow the connection between first shaft 15 and second shaft 26, so that second shaft 26 is allowed to make unitary rotation with first shaft 15. With the unitary rotation of first shaft 15 and second shaft 26, outer rotor 12 having the internal gear engaged with the external gear of pump rotor 11 of first pump element 1 and outer rotor 14 having the internal gear engaged with the external gear of pump rotor 13 of second pump element 2 are allowed to rotate, so that oil is sucked through suction port 39 into the suction side volume chambers of first pump element 1 and sucked through suction port 43 into the suction side volume chambers of second pump element 2. Subsequently, the suction side volume chambers are transferred and changed to the discharge side volume chambers that are reduced in volume to thereby increase the internal pressure as pump rotors 11, 13 meshed with outer rotors 12, 14 respectively are rotated. The oil sucked is discharged from the discharge side volume chambers into discharge ports 40, 44. Such a pump action is continuously performed to thereby successively feed the pressurized oil by rotation of pump rotors 11, 13 meshed with outer rotors 12, 14 respectively. Further, owing to such a liquid sealing effect that hermetic sealability of the respective volume chambers is enhanced by the sucked oil, there occurs a remarkable pressure difference between these volume chambers, thereby obtaining the pump action. In contrast, in a case where oil viscosity is higher than the predetermined value, the torque applied to clutch 7 is larger than the set torque so that clutch 7 is brought into the disengaged state to thereby interrupt the connection between first shaft 15 and second shaft 26. As a result, second pump element 2 is stopped, and only first pump element 1 is operated.

[Suppression of Heat Generation in Electric Motor Under Low Temperature Environmental Condition]

Usually, a gear pump of a trochoid type is a fixed-volume pump that discharges a constant amount of oil per unit rotation (one rotation). Therefore, torque necessary to operate the pump is increased as oil viscosity becomes higher. The conventional oil pump as an auxiliary equipment of an engine is driven by the engine. Therefore, the increase in required torque of the pump which is caused in accordance with a rise in oil viscosity will give a less influence upon the torque generated by the engine. However, in an electric oil pump that is driven by an electric motor, the increase in operation torque causes an increase in current consumption, thereby causing restriction (thermal restriction/current drive restriction) on operation of the driver.

As shown in FIG. 6A and FIG. 6B, in the electric oil pump, since the oil is discharged through the same pipe at a constant flow rate, required torque of the electric motor is increased as oil viscosity (cc oil temperature) becomes higher, and a current amount to be supplied to the electric motor is also increased. That is, the oil temperature range in which the electric oil pump can be operated is determined by allowable current and torque constant of the electric motor and the driver, theoretical discharge amount of the pump, and temperature dependency of oil viscosity. The amount of heat generated in the electric motor is increased in proportion to the square of current consumption of the electric motor. Therefore, in order to reduce the current consumption to not larger than the allowable current under a low temperature environmental condition where the oil viscosity is high, it is necessary to increase current drive tolerance of the driver. As a result, upsizing of the driver is caused, and expansion in installation space for meeting with a heat radiation design of the electric motor and selection of parts to allow a high current flow are required, which results in high costs.

In contrast, the electric oil pump according to the first embodiment includes first pump element 1 and second pump element 2 which are driven by electric motor 3, and clutch 7 that is disposed on a torque transmission path extending between electric motor 3 and second pump element 2 (between first shaft 15 and second shaft 26) and is actuated to allow torque transmission between electric motor 3 and second pump element 2 when oil viscosity is not higher than the predetermined value, and interrupts the torque transmission between electric motor 3 and second pump element 2 when the oil viscosity is higher than the predetermined value. That is, under an ordinary temperature environmental condition in which oil viscosity is not higher than the predetermined value, both first pump element 1 and second pump element 2 are actuated to thereby increase a discharge amount per one rotation of first shaft 15. Specifically, under an ordinary temperature environmental condition in which oil viscosity is low, a flow amount necessary to cool the automatic transmission, hold pressures of the respective friction engagement elements and carry out heat radiation or cooling of the respective friction engagement elements is large, and an oil leak amount is large. Therefore, a necessary discharge amount of the electric oil pump can be ensured by the two pump elements 1, 2. At this time, the driving load of electric motor 3 is small, and therefore, the heat generated in electric motor 3 is small.

On the other hand, under a low temperature environmental condition in which oil viscosity is higher than the predetermined value, second pump element 2 is disconnected from first pump element 1 to be kept in a free state, thereby reducing the discharge amount per one rotation of first shaft 15. Specifically, under a low temperature environmental condition in which oil viscosity is high, the driving load (required torque) of electric motor 3 is reduced as compared to that under the ordinary temperature environmental condition, thereby suppressing a current draw of electric motor 3. As a result, heat generation in electric motor 3 can be suppressed. At this time, a theoretical discharge amount of the electric oil pump is decreased in accordance with stop of second pump element 2, but the torque necessary to actuate the electric oil pump is reduced as compared to the case where both first pump element 1 and second pump element 2 are actuated. Therefore, the pump rotation speed under the same voltage condition can be increased to thereby compensate a part of reduction of the theoretical discharge amount. Further, under the low temperature environmental condition in which oil viscosity is high, a flow amount necessary to cool the automatic transmission and hold pressures of the respective friction engagement elements is small, and an oil leak amount is small. Therefore, even when a discharge amount of the electric oil pump is reduced as compared to that under the ordinary temperature environmental condition, a necessary discharge amount of the electric oil pump can be ensured. As shown in FIG. 7, in a case where both first pump element 1 and second pump element 2 are actuated under the low temperature environmental condition, when output torque of electric motor 3 is T₁₊₂, current consumption of electric motor 3 is I₁ and rotation speed of first shaft 15 is N₁. However, by stopping second pump element 2, although the output torque of electric motor 3 is reduced to T₁ and the current consumption of electric motor 3 is reduced to I₂, the rotation speed of first shaft 15 is increased to N₂.

In the electric oil pump according to the first embodiment, since heat generation from electric motor 3 under the low temperature environmental condition in which oil viscosity is high can be suppressed, the electric oil pump according to the first embodiment has the following advantages 1) to 3) as compared to the case where one fixed-volume pump is actuated.

1) Downsizing of electric oil pump: Since a necessary torque under the low temperature environmental condition becomes small, a current load of driver 5 can be reduced so that a relatively small FET, capacitor, etc. can be employed. Accordingly, it is possible to downsize driver 5. Since the amount of heat generated from an electric motor is increased in proportion to the square of the amount of current applied to the electric motor, a heat radiation area of electric motor 3 can be remarkably decreased by reducing the current consumption of electric motor 3, so that electric motor 3 can be downsized. 2) Cost down: Owing to reduction of the amount of current applied to electric motor 3, a diameter of a harness can be reduced, and parts such as a connector terminal can be simplified. As a result, it is possible to reduce the system cost. 3) Increase in reliability: Since generally fatigue life is varied in proportion to the square of a rate of temperature rise, thermal stress that is caused due to the heat generated from electric motor 3 can be remarkably reduced by suppressing the heat generation from electric motor 3. As a result, it is possible to increase reliability of the electric oil pump.

[Ensuring Discharge Characteristic]

In the electric oil pump according to the first embodiment, suction port 39 of first pump element 1 and suction port 43 of second pump element 2, and discharge port 40 of first pump element 1 and discharge port 44 of second pump element 2 are arranged in parallel with each other with respect to an oil circulation path extending between the oil pan and the control valve unit. For instance, as shown in FIG. 8A, in a case where first pump element 1 and second pump element 2 are arranged in series with respect to the oil circulation path and both of the two pump elements 1, 2 are actuated, the hydraulic pressure can be increased in respective pump elements 1, 2, and the hydraulic pressure increased can be discharged from respective pump elements 1, 2. On the other hand, as shown in FIG. 8B, in a case where first pump element 1 and second pump element 2 are arranged in series with respect to the oil circulation path and second pump element 2 is disconnected from first pump element 1, second pump element 2 generates discharge resistance to thereby lower the effect of reducing the output torque of the electric motor which can be attained by the disconnection of second pump element 2.

In contrast, in the electric oil pump according to the first embodiment, as shown in FIG. 9A, suction port 39 of first pump element 1 and suction port 43 of second pump element 2 are arranged in parallel with each other with respect to the oil circulation path, and discharge port 40 of first pump element 1 and discharge port 44 of second pump element 2 are arranged in parallel with each other with respect to the oil circulation path. With this arrangement, even when second pump element 2 is stopped as shown in FIG. 9B, it is possible to prevent second pump element 2 from generating the discharge resistance. Accordingly, the effect of sufficiently reducing the output torque of electric motor 3 can be attained to thereby more increase the rotation speed of first shaft 15 and ensure the discharge characteristic of the electric oil pump.

[Simplifying Pump Construction by Clutch]

In the electric oil pump according to the first embodiment, clutch 7 is disposed between first shaft 15 and second shaft 26, and the set torque at which clutch 7 is disengaged is set to the value of torque which is applied between first shaft 15 and second shaft 26 when oil viscosity becomes the predetermined value. Since clutch 7 is a torque sensitive clutch that is actuated using a difference between the biasing force of coil spring 7 e which acts on ball 7 d and a component of the force acting on ball 7 d from engaging portion 7 c, a sensor and a controller which detect and measure oil viscosity are unnecessary. Accordingly, it is possible to realize such a construction that performs connection and disconnection of second pump element 2 in accordance with oil viscosity with a reduced cost. Further, the construction is thus simplified to thereby serve to downsize the electric oil pump.

Effects of the electric oil pump according to the first embodiment will be described as follows.

(1) The electric oil pump includes first pump element 1 that is driven to suck and discharge oil by output torque of electric motor 3, second pump element 2 that is driven to suck and discharge the oil by the output torque of electric motor 3, and clutch 7 disposed on a torque transmission path extending between electric motor 3 and second pump element 2 (between first shaft 15 and second shaft 26). Clutch 7 is actuated to allow the torque transmission between electric motor 3 and second pump element 2 when viscosity of the oil is not higher than a predetermined value, and interrupt the torque transmission between electric motor 3 and second pump element 2 when viscosity of the oil is higher than the predetermined value. With this construction, it is possible to suppress heat generation from electric motor 3 under a low temperature environmental condition. (2) Suction port 39 of first pump element 1 and suction port 43 of second pump element 2 are arranged in parallel with each other with respect to the oil circulation path, and discharge port 40 of first pump element 1 and discharge port 44 of second pump element 2 are arranged in parallel with each other with respect to the oil circulation path. With this construction, it is possible to sufficiently attain the effect of reducing the output torque of electric motor 3 by stopping second pump element 2, and thereby ensure the discharge characteristic of the electric oil pump. (3) Clutch 7 is a torque sensitive clutch that is engaged when the torque applied between first pump element 1 and second pump element 2 is not higher than a set torque, and is disengaged when the torque applied between first pump element 1 and second pump element 2 is higher than the set torque. With the construction of clutch 7, it is possible to realize such a construction that performs connection and disconnection of second pump element 2 in accordance with oil viscosity with a reduced cost. Further, this construction is simplified, thereby serving to downsize the electric oil pump.

Second Embodiment

FIG. 10 is a schematic diagram showing an electric oil pump according to a second embodiment which differs from the first embodiment in arrangement of first pump element 1 and second pump element 2. Like reference numerals denote like parts, and therefore, detailed explanations therefor are omitted. As shown in FIG. 10, first pump element 1 and second pump element 2 are arranged in parallel with each other with respect to first shaft 15. Casing 60 includes first casing 61 that accommodates electric motor 3, second casing 62 that accommodates first pump element 1, second pump element 2 and clutch 7, and a third casing 63 that defines a common suction port and discharge ports of first pump element 1 and second pump element 2 as explained later. First casing 61 includes motor accommodating portion 64 in which electric motor 3 is accommodated. Stator 17 is fixedly supported in motor accommodating portion 64. Disposed on the side of second casing 62 within first casing 61 are bearing portion 65 in which first shaft 15 is rotatably supported, and bearing portion 66 in which third shaft 71 is rotatably supported as explained later.

Second casing 62 includes gear accommodating portion 67 that accommodates a pair of power transmission gears 72, 73, first pump element accommodating portion 68 that accommodates first pump element 1, second pump element accommodating portion 69 that accommodates second pump element 2, and clutch accommodating portion 70 that accommodates clutch 7. Specifically, the pair of power transmission gears 72, 73 that cooperate with each other to transmit rotation of first shaft 15 to third shaft 71, are accommodated in gear accommodating portion 67. Power transmission gear 72 is connected to first shaft 15, and power transmission gear 73 is connected to third shaft 71. Third shaft 71 is arranged in parallel with first shaft 15. Pump rotor 11 of first pump element 1 which is connected with third shaft 71 is rotatably supported in first pump element accommodating portion 68. Disposed between first pump element accommodating portion 68 and gear accommodating portion 67 is bearing portion 74 in which third shaft 71 is rotatably supported. Pump rotor 13 of second pump element 2 is rotatably supported in second pump element accommodating portion 69. Clutch 7 is connected with second shaft 26 connected with pump rotor 13 of second pump element 2. Disposed between clutch accommodating portion 70 and second pump element accommodating portion 69 is bearing portion 75 in which second shaft 26 is rotatably supported.

Third casing 63 includes bearing portion 76 in which a tip end portion of third shaft 71 is rotatably supported, and bearing portion 77 in which a tip end portion of second shaft 26 is rotatably supported. Third casing 63 includes suction port (oil suction port) 78 common to first pump element 1 and second pump element 2, discharge port (oil discharge port) 79 of first pump element 1, and discharge port (oil discharge port) 80 of second pump element 2.

A function of the electric oil pump according to the second embodiment will be explained hereinafter.

[Operation of Electric Oil Pump]

When first shaft 15 is driven to rotate by applying electric current to electric motor 3, the rotation of first shaft 15 is transmitted to third shaft 71 through the pair of power transmission gears 72, 73, and pump rotor 11 connected to third shaft 71 is rotated. At this time, in a case where oil viscosity is not higher than the predetermined value, the torque that is applied to clutch 7 disposed between first shaft 15 and second shaft 26 is not larger than the set torque. Therefore, clutch 7 is kept in the engagement state, so that second shaft 26 is allowed to make unitary rotation with first shaft 15. With the unitary rotation of first shaft 15 and second shaft 26, outer rotor 12 having the internal gear engaged with the external gear of pump rotor 11 of first pump element 1 and outer rotor 14 having the internal gear engaged with the external gear of pump rotor 13 of second pump element 2 are allowed to rotate, so that oil is sucked through suction port 78 into the suction side volume chambers of first pump element 1 and the suction side volume chambers of second pump element 2. Subsequently, the suction side volume chambers are transferred and changed to the discharge side volume chambers that is reduced in volume to thereby increase the internal pressure as pump rotors 11, 13 meshed with outer rotors 12, 14 respectively are rotated. The oil sucked is discharged from the discharge side volume chambers into discharge ports 79, 80. Such a pump action is continuously performed to thereby successively feed the pressurized oil by rotation of pump rotors 11, 13 meshed with outer rotors 12, 14 respectively. Further, owing to such a liquid sealing effect that hermetic sealability of the respective volume chambers is enhanced by the sucked oil, there occurs a remarkable pressure difference between these volume chambers, thereby obtaining the pump action. In contrast, in a case where oil viscosity is higher than the predetermined value, the torque applied to clutch 7 is larger than the set torque so that clutch 7 is brought into the disengaged state to thereby interrupt the connection between first shaft 15 and second shaft 26. As a result, second pump element 2 is stopped, and only first pump element 1 is operated.

The electric oil pump according to the second embodiment can attain the following effect in addition to the effects (1) to (3) of the electric oil pump according to the first embodiment. Since first pump element 1 and second pump element 2 are arranged in parallel with each other with respect to first shaft 15, it is possible to reduce an axial length of the electric oil pump as compared to the case where first pump element 1 and second pump element 2 are arranged in series with respect to first shaft 15.

Third Embodiment

FIG. 11 is a schematic diagram showing an electric oil pump according to a third embodiment which differs from the first embodiment in that first pump element 1 and second pump element 2 have a suction port in common. As shown in FIG. 11, first pump element 1 and second pump element 2 have one common suction port 51. Suction port 51 is formed on a radial outside of outer rotors 12, 14 of first and second pump elements 1, 2 and extends from first casing 18 into second casing 19 in the axial direction of first and second pump elements 1, 2. Suction port 51 is communicated with an oil inlet opened into an oil pan (not shown). The electric oil pump according to the third embodiment can attain substantially the same effects as those of the electric oil pump according to the first embodiment.

The electric oil pump of the present invention is not limited to the above-described embodiments in which the electric oil pump is used in an automatic transmission, and may be applied to any other hydraulically operating apparatus. Even in such a case, same effects as those of the above-described embodiments can be attained.

This application is based on a prior Japanese Patent Application No. 2011-075717 filed on Mar. 30, 2011. The entire contents of the Japanese Patent Application No. 2011-075717 are hereby incorporated by reference.

Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments as described above. Further variations of the embodiments as described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims. 

1. An electric oil pump comprising: an electric motor; a first pump element that is driven to suck and discharge oil by output torque of the electric motor; a second pump element that is driven to suck and discharge the oil by the output torque of the electric motor; and a torque transmission switching member disposed on a torque transmission path that extends between the electric motor and the second pump element, the torque transmission switching member being actuated to allow torque transmission between the electric motor and the second pump element when viscosity of the oil is not higher than a predetermined value, and interrupt the torque transmission between the electric motor and the second pump element when viscosity of the oil is higher than the predetermined value.
 2. The electric oil pump as claimed in claim 1, wherein the first pump element and the second pump element have oil suction ports and oil discharge ports, respectively, the oil suction ports of the first pump element and the second pump element and the oil discharge ports of the first pump element and the second pump element being respectively arranged in parallel with each other with respect to an oil circulation path.
 3. The electric oil pump as claimed in claim 1, wherein the torque transmission switching member is a torque sensitive clutch that is engaged when torque applied between the first pump element and the second pump element is not higher than a set torque, and is disengaged when the torque applied between the first pump element and the second pump element is higher than the set torque.
 4. The electric oil pump as claimed in claim 1, further comprising a first shaft through which the first pump element is driven, the first pump element and the second pump element being arranged in series with respect to the first shaft.
 5. The electric oil pump as claimed in claim 1, further comprising a first shaft through which the first pump element is driven, the first pump element and the second pump element being arranged in parallel with each other with respect to the first shaft.
 6. The electric oil pump as claimed in claim 1, wherein the first pump element and the second pump element have a suction port in common.
 7. The electric oil pump as claimed in claim 1, further comprising a second shaft connected with the second pump element, the torque transmission switching member being disposed between the first shaft and the second shaft.
 8. The electric oil pump as claimed in claim 5, wherein the first pump element and the second pump element have a suction port in common.
 9. The electric oil pump as claimed in claim 5, further comprising a second shaft connected with the second pump element, a third shaft that is parallel with the first shaft and connected with the first pump element, and a pair of power transmission gears through which the third shaft is connected to the first shaft. 